Exploring Photoinduced Excited State Evolution in Heterobimetallic

Article pubs.acs.org/JPCB

Exploring Photoinduced Excited State Evolution in Heterobimetallic Ru(II)−Co(III) Complexes Korina Kuhar, Lisa A. Fredin, and Petter Persson* Chemistry Department, Theoretical Chemistry Division, Lund University, Box 124, SE-22100 Lund, Sweden S Supporting Information *

ABSTRACT: Quantum chemical calculations provide detailed theoretical information concerning key aspects of photoinduced electron and excitation transfer processes in supramolecular donor−acceptor systems, which are particularly relevant to fundamental charge separation in emerging molecular approaches for solar energy conversion. Here we use density functional theory (DFT) calculations to explore the excited state landscape of heterobimetallic Ru−Co systems with varying degrees of interaction between the two metal centers, unbound, weakly bound, and tightly bound systems. The interplay between structural and electronic factors involved in various excited state relaxation processes is examined through full optimizations of multiple charge/spin states of each of the investigated systems. Low-energy relaxed heterobimetallic states of energy transfer and excitation transfer character are characterized in terms of energy, structure, and electronic properties. These findings support the notion of efficient photoinduced charge separation from a Ru(II)−Co(III) ground state, via initial optical excitation of the Ru-center, to low-energy Ru(III)−Co(II) states. The strongly coupled system has significant involvement of the conjugated bridge, qualitatively distinguishing it from the other two weakly coupled systems. Finally, by constructing potential energy surfaces for the three systems where all charge/spin state combinations are projected onto relevant reaction coordinates, excited state decay pathways are explored.

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INTRODUCTION Electron transfer processes provide a unique opportunity to achieve charge separation in molecular systems, as well as to drive reduction and oxidation of redox active components such as transition metal ions. This makes electron transfer processes fundamentally important for a diverse set of chemical and biological systems including, e.g., photosynthesis, solar cells, and molecular electronics.1−7 In particular, natural photosynthesis’ dominance as life on earth’s energy source for the last couple of billion years provides inspiration for several emerging technologies; molecular approaches for solar energy conversion are promising as both potentially inexhaustible and inexpensive sources of renewable energy.8 Current research activities including several types of photovoltaic devices such as dyesensitized solar cells (DSSCs) and organic photovoltaics (OPVs) have demonstrated significant improvement in device efficiency over the last 10−20 years.9 There is also a rapidly growing interest in photocatalytic systems for direct solar fuel production, such as hydrogen generation from water splitting through so-called artificial photosynthesis.10−14 The quest for efficient devices for solar energy conversion is paralleled by significant experimental and theoretical efforts to better understand and control photoinduced electron transfer (PET) processes in a wide variety of relevant supramolecular and heterogeneous systems.15,16 In general, these photoinduced processes involve complicated excited state dynamics that depend on both structural and electronic effects, and are © XXXX American Chemical Society

frequently further complicated by solvent interactions and/or coupling between the electronic and nuclear processes.3,5 Greater understanding of photoinduced electron transfer has been accomplished in the last few decades through advances in time-resolved optical spectroscopy, where ultrafast laser spectroscopy now provides standard tools to investigate the evolution of electronically excited states on picosecond and femtosecond time scales relevant for many PET processes.10,17 Such experiments have been successfully combined with theoretical calculations of key electronic factors, e.g., electronic coupling strengths, in order to provide significant insight into important processes such as long-range electron transfer in supramolecular and heterogeneous donor−bridge−acceptor systems.10,18−28 Very recently, independent advances in experimental and theoretical capabilities have combined to dramatically enhance avenues for investigating photoinduced structural evolution in relevant molecular systems. Recently, time-resolved X-ray spectroscopy has, in particular, been used to probe experimentally photoinduced structural evolution of transition metal complexes relevant for solar energy conversion applications.29−31 Recent progress in quantum chemistry, in Special Issue: John R. Miller and Marshall D. Newton Festschrift Received: October 31, 2014 Revised: February 21, 2015

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DOI: 10.1021/jp510950u J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

form a charge separated Ru(III)Co(II) state in which the catalytically active Co(II) oxidation state has been formed.55−60 It should also be noted that the reverse process, i.e., backelectron transfer corresponding to a final ground state regeneration step from Ru(III)−Co(II) to reform the Ru(II)−Co(III) ground state in our systems, is related to the use of Co(II)/Co(III) as a redox mediator in dye-sensitized solar cells that incorporate Ru-polypyridyl or other dye molecules as light-harvesters,33,61−69 which has also recently been investigated theoretically.61 The approach adopted here is to correlate photoinduced structural and electronic changes in the heterobimetallic systems beyond the initially excited Franck−Condon region through calculations of structurally and electronically relaxed excited triplet and quintet states formed following electron transfer (ET) or excitation energy transfer (EET), as illustrated in Scheme 1. This provides a theoretical framework for understanding the interplay between the structural and electronic factors involved in various excited state evolution processes.

particular with density functional theory (DFT) and timedependent DFT (TD-DFT) making accurate calculations of ground and excited state properties of a wide range of large molecular systems, including transition metal complexes, possible,32−36 means that experimental findings can now also be complemented theoretically. Quantum chemical calculations of excited potential energy surfaces beyond the initially excited Franck−Condon region have recently provided a more comprehensive theoretical understanding of the structural evolution accompanying photoinduced electronic processes for prototype light-harvesting transition metal complexes.37−42 The significant opportunities emerging for comprehensive characterization of the structural changes that accompany photoinduced processes point to the importance of expanding such investigations to more complex bimetallic systems that serve as prototype systems for understanding photoinduced electron transfer processes in donor−acceptor systems. Covalently linked bimetallic complexes have been used extensively to investigate electron transfer properties of donor− acceptor systems.43,44 This includes studies of bridged Ru(II)− Co(III) systems using traditional photoelectrochemical45 and recent X-ray spectroscopic techniques,46,47 as well as theoretical intermolecular electron transfer studies.48 In this study, we use first-principles quantum chemical calculations to theoretically investigate the excited state properties of heterobimetallic Ru(II)Co(III) complexes. The calculated systems are shown in Chart 1, and include the

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COMPUTATIONAL METHODS

Singlet, triplet, and quintet minima have been investigated using density functional theory (DFT) calculations employing the standard Stuttgart−Dresden (SDD) effective core potential (ECP) and Gaussian type orbital (GTO) basis set of double-ζ quality, 6-31G(d,p). Fully optimized geometries of the ground state (GS) and excited states were found. These optimizations were performed using the Gaussian 09 program,74 with the 6-31G(d,p) basis set for light atoms and SDD ECP for both metals, Ru and Co,70 the PBE071−73 hybrid functional, and a polarizable continuum model (PCM) description of an acetonitrile solvent environment. This level of theory is similar to the ones previously used for related tris(bidentate)ruthenium(II) complexes, which have been successfully investigated computationally in recent years.32,33,49 No symmetry was imposed in the optimization of the structures. Ground state properties have been calculated using the spin-restricted singlet formalism, while spinunrestricted DFT (uDFT) calculations have been performed for the lowest triplet and quintet state calculations. All optimized minima have spin contamination less than 1% (Table S2, Supporting Information). The time-dependent formulation of DFT (TD-DFT) including the PCM solvent was used to probe the absorption properties of the complexes. PBE0 was chosen for its superior geometric structures compared to many crystal structures of transition metal centered complexes, as well as the better match to experimental absorption spectra observed in previous studies,75−77 including for Ru complexes with extended side groups which are one step toward our bimetallic complexes from the extensively studies simple Ru-core complexes.75−77 In addition the single point energies of each optimized minimum were calculated with M06 and CAM-B3LYP as well as the first 10 TD-DFT transitions with CAM-B3LYP to check for signs of spurious low-energy CT excitations that could potentially arise in the bimetallic systems.

Chart 1. Structure of [Ru(bpy)2(dimethyl-bpy)]2+ or Ru (A), [Co(bpy)2(dimethyl-bpy)]3+ or Co (B), [Ru(bpy)2(dimethyl-bpy)Co(bpy)2(dimethyl-bpy)]5+ or RuCo (C), and [(bpy)2Ru(tpphz)Co(bpy)2]5+ or Ru Co (D)a

a

bpy = 2,2′-bipyridine, dimethyl-bpy = 4,4′-dimethyl-2,2′-bipyridine, and tpphz = tetrapyrido[3,2-a:2′,3′-c:3″,2″-h:2″,3‴-j]phenazine.

separate mononuclear complexes (Ru and Co) as the initial reference for a noninteracting unbound system ([Ru|Co]), as well as one weakly bound (RuCo)46 and one strongly bound (RuCo)47 heterobimetallic complex. These systems are of interest as model photoactive donor−acceptor dyad systems comprised of one light harvesting Ru complex41,42,49−54 coupled to a Co catalyst. Efficient light absorption of visible light by the Ru chromophore provides the necessary energy to drive a cascade of excited state processes that include photoinduced electron transfer from Ru(II) to Co(III) to

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RESULTS Combined Mononuclear Complexes, Unbound [Ru| Co]. Ground state optimizations of the mononuclear complexes, [Ru(bpy)2(dimethyl-bpy)]2+ (where bpy = 2,2′-

B


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The Journal of Physical Chemistry B Scheme 1. Photoinduced Electronic and Excited State Relaxation Processes in Donor−Acceptor (D−A) Systemsa

a Initial photoexcitation (Exc of energy Eexc) of the D−A ground state to an electronically excited donor state (D*−A) provides a driving force for conversion to energy transfer states (D−A*) and charge separated states (D+−A−) through excitation energy transfer (EET) and electron transfer (ET) processes. The excitation cycle is completed through ground state recovery (Rec) typically occurring from the lowest energy excited state. The population of different excited electronic states (left) is associated with characteristic structural relaxations toward excited state minima along multidimensional reaction coordinates (right) for electron transfer (QET) and excitation energy transfer (QEET).

Table 1. Calculated Properties for Optimized Structures of Mononuclear Ru and Co Complexes from PBE0/SDD[Ru,Co]/631G[N,H,C]/PCM(MeCN)a E (eV) Ru(II) 3 Ru(II) 2 Ru(III) 4 Ru(III) Co(III) 3 Co(III) 2 Co(II) 4 Co(II)

0 2.10 5.65 8.52 0 1.08 −4.89 −5.45

Mulliken spin density

Rb (Å)

Oc

qmaind

qminore

± ± ± ± ± ± ± ±

6.660 8.111 8.149 8.318 4.612 5.940 6.028 7.704

2.071 2.071 2.071 2.074 1.941 2.012 2.115 2.148

2.071 2.072 2.067 2.076 1.941 2.001 2.043 2.151

2.071 2.071 2.068 2.075 1.941 2.005 2.067 2.150

1.009 1.025 1.038 1.946 0.920 2.768

0.001 0.022 0.004 0.004 0.002 0.077 0.075 0.005

Distances in Å and angles in deg. Deviations are calculated as σn values. bR is the average of all metal coordinating atom bond distances. caverage deviation = (Σ|ideal angle(90°) − measured angle|)/n. dqmain is the average M−N bond length along the axis of most change, Ru(7−9) or Co(1−6). e qminor is the average M−N bond length along the axes of least change Ru(8−10,11−12) or Co(2−4,3−5). The M06 and CAM-B3LYP energy of each of these optimized minima is available in Table S3 (Supporting Information). a

Figure 1. Calculated first three HOMO and LUMO levels of Ru (left) and Co (right) with PBE0/SDD[Ru,Co]/6-31G[N,H,C]/PCM(MeCN). C


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the oxidized Ru(III) states after the removal of an electron are similar to other calculated oxidation energies.78,79 The fully optimized oxidized, reduced, and triplet states afford additional information to the traditional Franck− Condon picture, indicating the changes in geometry required to stabilize these types of states. In addition, the energies and spin densities (Figure S1, Supporting Information) of these optimized minima indicate how much energy is lost as a consequence of the geometric reorganization, i.e., compared to the approximate energies of these states based on the electronic structure at the GS geometry, as well as where in the molecule the unpaired electrons are most stable. As typical of Ru(II) complexes, the optimized triplet near the GS geometry is MLCT-like. For Co, the optimized 3Co(III) is calculated to be only 1.08 eV above the singlet GS. The Co optimized triplet is metal centered in nature with both unpaired electrons on the Co center (Figure S2, Supporting Information). Reduction of the cobalt site to form a Co(II) state is important for chargeseparation processes in the heterobimetallic systems. As expected, the optimized 2Co(II) and 4Co(II) have negative energies with respect to the GS (Table 1) due to the favorable addition of an electron to the Co atom. The relaxed doublet and quartet are also metal centered (Figure S2, Supporting Information), in contrast to the Co ground state LUMO, which indicates that excitation of an electron from the HOMO to the LUMO would localize the unpaired electron on the ligand system and the HOMO would indicate a hole left also on the ligands. However, all of the spin densities of the relaxed 3 Co(III), 2Co(II), and 4Co(II) (Figure S2, Supporting Information) show localization of unpaired electrons on the Co center. Utilizing the energies of all the relaxed structures of the mononuclear complexes, the unbound [Ru|Co] D−A system has been constructed where the charge/spin state of each site and the overall spin state are indicated by the notation m3 m1 [ Ru(Q1)|m2Co(Q2)], where m1, m2, and m3 are the multiplicity of the Ru site, Co site, and overall complex, respectively, and Q1 and Q2 are the charge on the Ru and Co sites, respectively, such that Q1 + Q2 = +5. Multiple spin state combinations compatible with general expectations for this kind of D−A system have been mapped in a Jablonski-like diagram, Figure 2, where the horizontal lines represent the sum

bipyridine and dimethyl-bpy = 4,4′-dimethyl-2,2′-bipyridine) or Ru and [Co(bpy)2(dimethyl-bpy)]3+ or Co (Chart 1A and B), have been performed. Selected structural results are summarized in Table 1 and are presented as two central structural parameters (R− and O−). The R value is the average metal− ligand bond length for the first coordination sphere with a standard deviation error. The O value (octahedricity value) is a measure of the mean absolute deviation of the set of all metal ligand bond angles from their “ideal” octahedral values (ideal being O of 0).41 Figure 1 shows the three highest occupied molecular orbitals (HOMOs) and three lowest unoccupied molecular orbitals (LUMOs) for both complexes. As expected,51−53 the calculated electronic structure of Ru contains a set of three closely spaced metal (Ru 4d) t2g levels as the highest occupied MOs, while all three lowest unoccupied MOs have mainly ligand π* character. The HOMO and LUMO energies of the Co ground state (GS) are both lower than the corresponding Ru levels. As seen in previous pyridyl Co(II) complexes,61 the three highest occupied MOs are on the ligands, HOMO−1 and HOMO−2 on both pure bpy ligands while HOMO is localized on the dimethyl-bpy but no influence of methyl groups has been detected. In addition to the HOMOs which have been previously characterized,61 here we examine the Co LUMOs. The Co LUMO shows combined metal d and π* character involving two out of the three bpy ligands, whereas LUMO+1 and LUMO+2 involve contributions from all three ligands. The electronic properties of an unbound, and thus essentially electronically decoupled, heteronuclear bimetallic system, [Ru| Co], are considered by combining the separate properties of the two mononuclear complexes. A combination of the two MO pictures in Figure 1 indicates that the HOMO of the combined system is the Ru HOMO, while the combined [Ru| Co] LUMO is the Co LUMO. This gives a calculated HOMO−LUMO energy difference for the combined system of 3.5 eV. Further information about excited state decay pathways in the [Ru|Co] model donor−acceptor system beyond a static electronic coupling perspective can be obtained by computationally characterizing the structurally relaxed properties of different excited states corresponding to excited donor (D*− A), excitation energy transfer (D−A*), and electron transfer (D+−A−) states, as outlined in Scheme 1. This is done for the decoupled case through optimizations of several different lowenergy charge/spin states of the mononuclear Ru and Co complexes, with results for the investigated states listed in Table 1. In the donor−acceptor (D−A) model of the heterobimetallic Ru−Co system, Ru plays the role of the electron donor, so the formation of an oxidized Ru(III) state following excitation of the ruthenium complex, Ru(II)*, is expected. The energy of the lowest structurally relaxed relevant high spin state, 3Ru(II), is calculated to be 2.10 eV higher than the Ru GS and is a metalto-ligand charge transfer triplet, where one unpaired electron is on the Ru center and one is on the ligands. The individual triplet Ru(II) states in this study are all MLCT in nature, since the short-lived metal-centered (MC) states41,42 are not thought to be directly involved in energy or electron transfer to an acceptor. Two oxidized state minima have been optimized, 2 Ru(III) and 4Ru(III). As expected, the calculations yield the doublet 2Ru(III) state to be substantially lower in energy compared to the quartet 4Ru(III) state and the high energies of

Figure 2. Energy diagram of decoupled [Ru|Co] states formed from combining various charge/spin states of the mononuclear constituents shown at the maximal total system spin. D


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The Journal of Physical Chemistry B of the calculated energies for the two relaxed constituent minima states (Table S1, Supporting Information) and the columns correspond to the (maximum) overall spin of the [Ru| Co] system: singlet, triplet, or quintet. A singlet D−A ground state of the combined system can be constructed from the individual 1Ru(II) and 1Co(III) ground states, both with energy set to zero, forming 1[1Ru(II)|1Co(III)] where the energy of this state is also zero. The excited 1 Ru(II)* (at the GS geometry) is 2.73 eV, calculated by TDDFT, and, when combined with the Co GS, forms an excited singlet 1[1Ru(II)*|1Co(III)], with all the excited state energy coming from the excited Ru. The Ru excited state that is most often probed experimentally is an MLCT 3Ru(II), calculated at 2.10 eV, which combined with the Co GS gives 3[3Ru(II)|1Co(III)] that corresponds to an initial D*−A state. In comparison, the Ru GS, i.e., 1Ru(II), and 3Co(III) can form a low-energy 3 1 [ Ru(II)|3Co(III)] state at 1.08 eV that corresponds to a structurally relaxed excitation energy transfer state, D−A*. The lowest energy triplet is, however, calculated to result from a combination of the oxidized 2Ru(III) and reduced 2Co(II) species, resulting in a charge separated D+−A − state characterized as 3[2Ru(III)|2Co(II)] at 0.76 eV. A low-energy quintet state can be formed as a doublet-quartet combination, 5 2 [ Ru(III)|4Co(II)] state formed by 2Ru(III) and 4Co(II), giving a low lying quintet, at 0.2 eV, or conversely 4Ru(III) and 2 Co(II) can combine to form a 5[4Ru(III)|2Co(II)] state which, however, has an energy of 3.07 eV that is much higher than the initial excited singlet and thus not relevant for the excited state relaxation processes of interest here. The interplay between the unbound [Ru|Co] excited states suggests an excited state decay with a reduced Co(II) quartet as the lowest energy (vide inf ra). Weakly Bound Ru−Co. Properties of a weakly bound binuclear [Ru(bpy) 2 (dimethyl-bpy)−Co(bpy) 2 (dimethylbpy)]5+, or Ru−Co, complex (Chart 1C) have been calculated by connecting the mononuclear Ru and Co through a saturated alkyl bridge. In the ground state of the binuclear Ru−Co, the geometries around the two metal centers do not differ significantly from the corresponding ground state geometries of the mononuclear Ru and Co complexes. Since the saturated bridge does not distort the geometry of the two sites, the two metal centers retain their autonomy and show similar behavior to the corresponding mononuclear complexes. Thus, Ru−Co can be seen as three charge/spin states that combine to uniquely identify the system: the Ru charge/spin state comparable to Ru, the Co charge/spin state comparable to Co, and the overall system charge/spin state. The first three HOMOs of Ru−Co (Figure 3) are pure 4d orbitals on Ru; on the other hand, the Ru−Co LUMOs are essentially located on the Co site, analogous to the Co LUMOs with the metal d-orbitals mixing with the ligand π*. The electronic structure of the weakly bound Ru−Co correlates well with the properties inferred for the chemically unbound [Ru| Co] system with a HOMO−LUMO gap of 3.5 eV. An investigation of higher spin states in Ru−Co resulted in the identification of two different kinds of triplets and a quintet. Selected structural results for the full unrestricted excited state geometry optimizations are summarized in Table 2. The lower energy triplet is a charge separated 3CS state, above which a metal-to-ligand charge transfer 3MLCT state was found. The optimized quintet minimum is also charge separated in nature, 5 CS.

Figure 3. First three HOMO and LUMO levels of the weakly bound Ru−Co with PBE0/SDD[Ru,Co]/6-31G[N,H,C]/PCM(MeCN).

The 3MLCT excited state has the same Co-site geometry as the GS, but the Ru site has significant asymmetric stretching along two axes (defined as 8−11 and 10−12, Chart 1C), where the two Ru−N bonds along each axis shift in the same direction, shortening one Ru−N bond the same amount as the other is lengthened. Interestingly, this occurs on the two nonbridging axes, while the bonds on the bridging (7−9) axis do not show this kind of displacement. The type of excited state can be better understood by comparing the metal−N bond lengths in the weakly bound Ru−Co optimized minima to the unbound mononuclear Ru and Co (Figure 4). The mononuclear Co GS geometry agrees with the Co site of the Ru−Co 3MLCT, confirming that the Co site has not changed, while the noticeable structural changes on the Ru site are of similar magnitude but different (as defined) ligand axes, as the Ru MLCT triplet state that is dominated by a local Ru-site excitation, resulting in 3[3Ru(II)−1Co(III)] at 2.11 eV. In contrast, the other Ru−Co triplet minimum, a charge separated excited state 3CS, at 0.73 eV, is formed from a Ru site equivalent to the GS and a distorted Co site. The Co site experiences extensive stretching along the (1−6) bridging axis together with limited stretching of the remaining Co−N bonds. Tracking this (1−6) elongation as qCo (Figure 4A), the 3CS Co-site geometry is very similar to the 2Co(II) of the unbound mononuclear complex. The Ru site in Ru−Co 3CS is not uniquely determined by the geometry, since the Ru-site changes are not significant for any state other than the Ru 3 MLCT. The calculated Mulliken spin density values on the metal centers in the 3CS state (Table 3) are both ∼1; the spin densities (Figure 5) show the existence of spin density on each metal center with very little mixing with the ligands. The two unpaired spins are evenly spread across both metal centers, making each site a doublet. An overall triplet, formed from two doublets, indicates an electron transfer oxidizing the Ru(II) site forming a 2Ru(III) doublet and reducing the Co(III) site forming 2Co(II), resulting in 3[2Ru(III)−2Co(II)]5+, consistent with a typical D+−A− charge transfer state in a donor−acceptor model. E


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The Journal of Physical Chemistry B Table 2. Calculated Structural Properties of Ru−Co with PBE0/SDD[Ru,Co]/6-31G[N,H,C]/PCM(MeCN)a Rb geometry

E (eV)

GS 3 MLCT 3 CS 5 CS

0 2.11 0.72 0.18

Oc

Ru 2.0711 2.07 2.075 2.150

± ± ± ±

Co 0.0006 0.02 0.003 0.003

1.940 1.941 2.06 2.076

± ± ± ±

0.002 0.001 0.1 0.002

qmaind

qminore

Ru

Co

Ru

Co

Ru

Co

6.55 8.16 8.52 8.59

4.63 4.65 6.01 7.82

2.071 2.075 2.076 2.076

1.939 1.941 2.201 2.153

2.071 2.070 2.075 2.076

1.941 1.941 1.997 2.149

Distances in Å and angles in deg. Deviations are calculated as σn values. bR is the average of all metal coordinating atom bond distances. cAverage deviation = (Σ|ideal angle(90°) − measured angle|)/n. dqmain is the average M−N bond length along the axis of most change, Ru(7−9) or Co(1−6). e qminor is the average M−N bond length along the axes of least change Ru(8−10,11−12) or Co(2−4,3−5). The M06 and CAM-B3LYP energy of each of these optimized minima is available in Table S3 (Supporting Information). a

Figure 4. Calculated changes in Ru−N bonds (A) and Co−N bonds (B) in Ru−Co optimized geometries (red) compared to single Co and Ru geometries in different spin states (black) where each is graphed as qmain vs qminor as defined in Table 2. Note the changes around the Ru center are an order of magnitude smaller than those around Co, and significantly less than the typical ∼0.2 Å seen to stabilize Ru 3MC states.

Table 3. Calculated Energies and Spin States for Ru−Co Optimized GS, 3MLCT, 3CS, and 5CS Geometries, as Well as Corresponding Single Point Calculations for Alternative Spin States at Those Geometries with PBE0/SDD[Ru,Co]/ 6-31G(p,d)[N,H,C]/PCM(MeCN) Mulliken spin density geometry

spin

GS

S0 T1 Q1 S0 T1 Q1 S0 T1 Q1 S0 T1 Q1

3

3

5

MLCT

CS

CS

spin states

E (eV)

[ Ru(II)−1Co(III)]5+ 3 3 [ Ru(II)−1Co(III)]5+ 5 4 [ Ru(III)−2Co(II)]5+ 1 1 [ Ru(II)−1Co(III)]5+ 3 3 [ Ru(II)−1Co(III)]5+ 5 2 [ Ru(III)−4Co(II)]5+ 1 1 [ Ru(II)−1Co(III)]5+ 3 2 [ Ru(III)−2Co(II)]5+ 5 4 [ Ru(III)−2Co(II)]5+ 1 1 [ Ru(II)−1Co(III)]5+ 3 [Ru(II)−3Co(III)]5+ 5 2 [ Ru(III)−4Co(II)]5+

0 2.38 4.74 0.25 2.11 1.95 0.90 0.72 4.00 8.35 5.70 0.18

1 1

Ru

Co

1.020 2.583

0.000 0.872

1.011 1.018

0.000 2.637

1.026 2.604

0.940 0.945

0.000 1.027

2.421 2.768

The final calculated excited state minimum is a 0.18 eV charge separated quintet, 5CS. Geometrically, like the 3CS, the Ru site is not significantly changed from the GS. Meanwhile, the Co site experiences symmetrical elongation of all six Co−N

Figure 5. Triplet (left) and quintet (right) spin densities of Ru−Co at all optimized minima geometries. F


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The Journal of Physical Chemistry B bonds. This kind of deformation is characteristic of the high spin reduced 4Co(II) mononuclear state, and the geometry of the Co site in the Ru−Co 5CS is practically the same as 4Co(II) (Figure 4A). An overall quintet spin state and a Co-site quartet imply a Ru-site doublet; therefore, the Ru−Co 5CS state is assigned as 5[2Ru(III)−4Co(II)]5+. For each optimized minimum, the corresponding crossenergies, the energy of a different overall system spin state from that used to optimize the geometry, were calculated (Table 3), e.g., the system triplet single point energy at the ground state singlet geometry. This yields three energies for each optimized minimum: an overall system singlet, triplet, and quintet. By projecting the optimized minima and their corresponding crossenergies onto a well-chosen reaction coordinate, the energy of the system singlets, triplets, and quintets can be correlated to the charge/spin states of the sites and the geometric changes in the system. Using the Mulliken spin density values and spin density figures (Figure 5), it is possible to identify the individual spin state of each site for all cross energy states. The triplet energy at the GS geometry is a Ru based MLCT with all the spin density on the Ru site and none on the Co site, resulting in a 3[3Ru(II)−1Co(III)]5+, whereas the quintet of the same geometry is a charge separated state with a quartet on Ru and doublet on Co, 5[4Ru(III)−2Co(II)]5+. In addition, the quintet energy of the 3MLCT geometry is a charge separated quintet, 5[2Ru(III)−4Co(II)]5+. More importantly, this quintet is 0.16 eV below the 3MLCT minima, indicating that the two potential energy surfaces are very close at this geometry. The quintet spin state at the 3CS minimum geometry is a higher energy charge separated quintet, with a Ru quartet and Co doublet, 5[4Ru(III)−2Co(II)]5+. Finally, the only pure Co excited state resulting from excitation energy transfer, forming a D−A* state, is a Co MLCT triplet spin state at 5.7 eV at the 5 CS minimum geometry. By plotting the Ru−Co excited states along the reaction coordinate that undergoes the most change, qCo, (Figure 6), including all the optimized minima points and their corresponding cross-energies, and by connecting energies of like charge/spin states, a projected potential energy surface (PPES) picture is constructed. Projecting all calculated points onto the relevant coordinate, qCo, the PPES picture gives an indication of the interplay between states, it is important to remember that this projection simplifies all the complicated degrees of freedom in the fully relaxed minima to a single coordinate. On the basis of the total information from the PPES including both the positions of the relaxed minima that have been found and the identification of cross-energies for other spin states at these minima, it is possible to consider different excited state relaxation processes in Ru−Co in some further detail. First, the presence of the 3MLCT geometry close to the GS supports the singlet excitation of the Ru site and a possible intersystem crossing (ISC) to the Ru 3MLCT minimum, as has been seen in most mononuclear Ru(II) complexes. Second, the lowest energy excited state is the 5CS state with a Co quartet. From these observations and the PPES (Figure 6), a few excited state pathways can be suggested. After the initial excitation and relaxation of the Ru site resulting in the 3MLCT, the 5[2Ru(III)−4Co(II)]5+ quintet state at the 3MLCT geometry is energetically just below the triplet minimum, indicating potential further ISC to the quintet surface. In this case, the decay to the 5CS is likely, since the high spin Co-site quartet will force the system to find a more stable geometry by pushing the ligands outward. The PPES,

Figure 6. Projected potential energy surface (PPES) for Ru−Co where all calculated points are projected onto the qCo axis, which is the main Co reaction coordinate, the average M−N bond length along the axis of most change, Co(1−6). Red points are optimized minima, and black points are single point energies calculated at the minimum geometries; ●, ∗, and + are singlets, triplets, and quintets, respectively. The gray lines schematically show the potential energy surfaces, and the inset indicates the differences between calculated minima geometries in both main q’s as defined in Figure 4. Note the large difference in energy of the Q1 at the GS and 3MLCT geometry is due to the different natures of these two states which emphasizes the differences in these geometries that is minimized in this projection.

however, also suggests the presence of competing pathways, including either a return to the GS surface before reaching the 5 CS minimum or a charge transfer from the 3MLCT to the 3CS before ISC to the 5CS. An ISC between the 3MLCT and 5CS surfaces would result in the simultaneous contraction of bonds on the Ru site as it goes from 3Ru(II) to 2Ru(III) and the significant elongation of all Co−N bonds as the Co site gains two unpaired electrons. Instead, an electron transfer followed by an ISC would result in simultaneous contraction in bonds on the Ru site as it goes from 3Ru(II) to 2Ru(III) and relaxation along one axis of the Co site while forming a doublet after receiving an electron from the Ru site, followed by an ISC of the Co to the quartet, lowering its total energy. Even for the Ru−Co states in which charge transfer occur, the individual site’s properties correspond well to the uncoupled [Ru|Co]. Therefore, the calculations support the notion that the Ru−Co bimetallic system characterized by a saturated alkyl bridge is in fact well-described as an electronically weakly coupled system. Strongly Bound RuCo. Formation of a strongly bound complex has been achieved experimentally by introducing a conjugated bridge.47 Envisioning starting from the unbound pair, the dimethyl-bpy ligands are linked via an aromatic ring forming [(bpy)2Ru(tpphz)Co(bpy)2]5+ or RuCo (Chart 1D) (where tpphz = tetrapyrido[3,2-a:2′,3′-c:3″,2″-h:2″,3‴j]phenazine). The strongly delocalized bridging ligand is flat and therefore connected to both metal centers in the same plane. The ground state optimization of RuCo indicates that, even though the bridge strongly connects the two sites, the G


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Ru (Table 4) correspond well with a Ru 3MLCT state, like that in the weakly bound RuCo. In RuCo, the spin density (Figure 8) is localized extensively across the whole bridging

geometry of each site is similar to the corresponding uncoupled mononuclear GS. However, the ground state electronic structure differs from the weakly bound RuCo in new purely bridge localized molecular orbitals. The first three HOMO levels of RuCo (Figure 7) are again essentially Ru

Figure 7. Three highest occupied and three lowest unoccupied MOs of the strongly bound RuCo according to PBE0/SDD[Ru,Co]/631G[N,H,C]/PCM(MeCN).

centered atomic d-orbitals with only minor contribution from the surrounding ligands. The first RuCo LUMO is a fully bridge localized π*-orbital; the next two LUMOs are typical Co LUMOs, seen previously in RuCo. The HOMO−LUMO gap has decreased to ∼3 eV, due to the low lying bridge π*. The first TD-DFT singlet excitation at 2.4 eV is a Ru based MLCT, where the ligand excitation is delocalized across the entire bridge ligand. Because of this, we continue with the useful notation of RuCo and a previous experimental paper46 with the bridge ligand assigned as a Ru ligand. The optimization of higher spin states led to five relaxed excited state minima, three triplets and two quintets whose structural parameters have been summarized in Table 4. Two different types of triplets and quintets, MLCTs and CSs, were found. The 3MLCT state is 1.79 eV, without significant distortion of either the Ru- or Co-site geometry. No spin density on the Co center and a Mulliken spin density of ∼1 on

Figure 8. Triplet (left) and quintet (right) spin densities of RuCo at all optimized minima geometries.

ligand, making the Ru site including the whole bridge a triplet, 3 3 [ {Ru(II)}1Co(III)]5+. The 4 eV quintet minimum, 5 MLCT, is also a Ru MLCT state with elongated RuN bonds. Although all the RuN bonds stretch outward, the most significant elongation is along the 9−12 axis where a RuN stretch of more than 0.2 Å is observed. The implication from the geometric changes is that the electron distribution is mainly on the Ru site which is essentially two excitations from the closed shell singlet, a combination of one MLCT excitation and one MC excitation at the Ru site leaving three unpaired

Table 4. Calculated Structural Properties of RuCo with PBE0/SDD[Ru,Co]/6-31G[N,H,C]/PCM(MeCN)a Rb geometry

E (eV)

GS 3 MLCT 5 MLCT 3 CS 5 CS 3 CS′

0 1.76 4.00 0.67 0.33 0.13

Oc

Ru 2.073 2.077 2.20 2.079 2.080 2.079

± ± ± ± ± ±

Co 0.005 0.003 0.12 0.009 0.007 0.009

1.945 1.943 1.944 2.07 2.15 2.15

± ± ± ± ± ±

0.008 0.005 0.003 0.11 0.01 0.02

qmaind

qminore

Ru

Co

Ru

Co

Ru

Co

6.50 8.11 10.85 8.27 8.17 8.17

4.47 4.36 4.31 5.94 6.85 6.85

2.076 2.078 2.188 2.084 2.083 2.083

1.947 1.945 1.945 2.209 2.164 2.157

2.072 2.076 2.211 2.077 2.078 2.078

1.944 1.942 1.944 1.999 2.144 2.151

a Distances in Å and angles in deg. Deviations are calculated as σn values. bR is the average of all metal coordinating atom bond distances. cAverage deviation = (Σ|ideal angle(90°) − measured angle|)/n. dqmain is the average M−N bond length along the axis of most change, Ru(7−9) or Co(1−6). e qminor is the average M−N bond length along the axes of least change Ru(8−10,11−12) or Co(2−4,3−5). The M06 and CAM-B3LYP energy of each of these optimized minima is available in Table S3 (Supporting Information).

H


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Table 5. Calculated Energetic and Spin States for RuCo Optimized GS, 3MLCT, 3CS, 5MLCT, and 5CS Geometries and Corresponding Single Point Calculations for Remaining Spin States at Those Geometries with PBE0/SDD[Ru,Co]/631G(p,d)[N,H,C]/PCM(MeCN) Mulliken spin density geometry

spin

GS

S0 T1 Q1 S0 T1 Q1 S0 T1 Q1 S0 T1 Q1 S0 T1 Q1 S0 T1 Q1

3

MLCT

5

MLCT

3

CS

5

CS

3

CS′

spin states

E (eV)

[ Ru(II)1Co(III)]5+ 3 3 [ Ru(II)1Co(III)]5+ 5 1 [ Ru(II)5Co(III)]5+ 1 1 [ Ru(II)1Co(III)]5+ 3 3 [ {Ru(II)1Co(III)]5+ 5 4 [ {Ru(III)}2Co(II)]5+ 1 1 [ Ru(II)1Co(III)]5+ 3 3 [ {Ru(II)}1Co(III)]5+ 5 5 [ {Ru(II)}1Co(III)]5+ 1 1 [ Ru(II)1Co(III)]5+ 3 2 [ Ru(III)2Co(II)]5+ 5 4 [ {Ru(III)}2Co(II)]5+ 1 1 [ Ru(II)1Co(III)]5+ 3 2 [ Ru(II)4Co(III)]5+ 5 2 [ Ru(II)4Co(III)]5+ 1 1 [ Ru(II)1Co(III)]5+ 3 2 [ Ru(III)2Co(II)]5+ 5 2 [ Ru(III)4Co(II)]5+

0 1.93 2.36 0.14 1.76 3.26 1.66 3.12 4.00 0.87 0.67 3.72 1.54 0.17 0.33 0.13 0.13 1.55

1 1

Ru

Co

1.028 0.000

0.002 3.145

1.027 1.028

0.0014 1.8478

1.004 2.637

0.001 0.001

1.029 1.034

0.942 0.968

−1.029 1.030

2.773 2.754

−0.000059 1.029

2.421 2.770

Figure 9. Changes in RuN bonds (A) and CoN bonds (B) in RuCo optimized geometries (red) compared to single Co and Ru geometries in different spin states (black) where each is graphed as qmain vs qminor as defined in Table 4.

this is similar even with the conjugated bridge in RuCo allowing for direct comparison to better understand the role of the bridge. The fully optimized triplet, 3CS, at 0.67 eV shows no significant changes in the RuN bonds, and therefore, the geometry of the Ru site is much like the Ru GS. Conversely, the Co site undergoes major stretching along one axis (defined as the 1−6 axis), which is characteristic of the 2Co(II) state (Figure 9). In the RuCo 3CS, the bonds along the axis of most change, qCo, stretch more than 0.25 Å and the remaining four CoN bonds undergo small relaxation. The RuCo 3CS and mononuclear 2Co(II) show similar characteristic stretching. In addition to this characteristic one axis stretching, the

electrons on the Ru center and one on the bridging ligand, supported by the high Mulliken spin density value (Table 5) on the Ru indicating multiple unpaired electrons. The spin density (Figure 8) shows involvement of both the Ru center and bridge, making it a 5[5{Ru(II)}1Co(III)]5+. Again, the bridge acts like a Ru ligand which shows coupling of the two sites. The delocalized spin density on the bridge in both MLCT states displaces the unpaired electrons on the ligands toward the Co site, unlike RuCo where the opposite was true. The importance of electron transfer between sites prompted extensive analysis of the CS minima. In RuCo, the charge separated states underwent large distortion on the Co site due to the transfer of an electron from the Ru site to the Co site; I


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The Journal of Physical Chemistry B Mulliken spin density values confirm localization of charge on both Ru and Co (Figure 8) with little ligand spin density, in good correspondence with the CS states of the weakly coupled RuCo. Finally, the Co site in 3CS is identified as a reduced doublet, and since there is evidence of electron transfer, the Ru site is an oxidized doublet, to form an overall triplet, 3[2Ru(III)2Co(II)]5+. The quintet CS state, 5CS, at 0.33 eV shows no structural deformation of the Ru site (Figure 9A) and a symmetric stretching outward of all CoN bonds by ∼0.2 Å (Figure 9B), which is a perfect match with the mononuclear 4Co(II). The spin density distribution, both the Mulliken spin density values and figures (Figure 8), indicates the charge separated nature of this state. Since both the geometry and spin density show a reduced quartet on the Co site, the Ru site is assigned as an oxidized doublet to form an overall quintet, 5[2Ru(II)4Co(III)]5+. Overall, the conjugated, less flexible bridge restricts the freedom of movement at each site, limiting the rearrangement of the ligands. Consequently, the only major bond elongation on the Ru site occurs in the high spin 5MLCT state where the quintet centered on the Ru site causes significant outward stretching; for any calculated lower spin state, no structural changes are observed, indicating the more ridged ligand system in RuCo is able to stabilize up to three unpaired electrons centered on the Ru site with little structural relaxation. The largest Ru-site deformation is the elongation along the 9−12 axis (Chart 1D) in the 5MLCT, making this an obvious qRu; note in Figure 9B how all geometries except the 5MLCT are crowded around the GS. The majority of structural rearrangement in the excited states occurs on the Co site, and the bond elongations are very similar to those in RuCo. Significant stretching along one axis characteristic of a reduced 2Co(II) and symmetrical elongation of all bonds characteristic of a reduced 4Co(II) are both present in the excited states where an electron has been transferred and charge separation has occurred. In addition to the optimized minima, their alternative spin states, the cross-energies, help to understand the spin state surfaces of RuCo. In the same manner used for RuCo, the Mulliken spin density values of each of the metal two centers listed in Table 5 together with the spin density distributions shown in Figure 8 provide a basis to assign spin states also to the cross-energy spin states listed in Table 5; a detailed discussion of the assignment of each of the cross energies is available in the Supporting Information. A PPES above the qCo has been constructed (Figure 10) and gives an overview of the calculated states for RuCo. The two triplets match well in type, energy, and distance along the reaction coordinate, qCo, to those found for the weakly coupled RuCo. An additional optimized minimum quintet, 5MLCT, and a second 3CS′ state were found for RuCo. However, the main influence of the conjugated bridge is in the quintet surfaces, where four types of overall quintets were found in the relevant energy range, below 4 eV, 5[1Ru(II)5Co(III)]5+, 5 5 [ {Ru(II)}1Co(III)]5+, 5[4{Ru(III)}2Co(II)]5+, 5[2Ru(II)4Co(III)]5+, making any discussion of possible deactivation pathways very complicated. The major structural changes on the Co site occur upon charge transfer from the Ru site to the Co site, resulting in a higher spin state Co, doublet or quartet, and a significantly stabilized overall charge separated state, 3CS, 3CS′, and 5CS. Analogous to the weakly bound RuCo, the lowest energy states are charge separated with a Co quartet, as seen in the experiments.46,47

Figure 10. Projected potential energy surface (PPES) for RuCo where all calculated points are projected onto the qCo axis, which is the main Co reaction coordinate, the average MN bond length along the axis of most change, Co(1−6). Red points are optimized minima, and black points are single point energies calculated at the minimum geometries; ●, ∗, and + are singlets, triplets, and quintets, respectively. The gray lines schematically show the potential energy surfaces, and the inset indicates the differences between calculated minima geometries in both main q’s as defined in Figure 9. Note overlapping T1 and Q1 energies in the GS, 3MLCT, and 5MLCT regions are due to differences in geometries in coordinates other than the projection axis.

The less flexible bridge restricts the freedom of movement at each site, playing a more significant role in the strongly bound RuCo, forming a strongly bound system with strong coupling of the metal centers.

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DISCUSSION The calculations of several different charge/spin states for the three systems provided in the Results section above provide a basis for understanding differences in the excited state evolution in these D−A model complexes characterized by different interactions between the two metal cores. The experimental46,47 and calculated (Figure 11) absorption spectra show that the low energy region, 350−660 nm, is dominated by a Ru MLCT peak, calculated to be centered around ∼420 nm in the Ru, RuCo, and RuCo spectra, matching very well to the experimentally observed Ru → bpy absorption in the 400− 440 nm region.47 In addition, the natural transition orbitals (NTOs)80 provide a method for visualizing the hole and electron of each computed transition, which is particularly useful for these types of large systems where traditional TDDFT excitations are the combination of many MO combinations; the NTOs provide a compact description of the excitation. This strong Ru MLCT peak shows that the majority of light absorption would occur at the Ru site, indicating its donor potential. A brief analysis and comparison of the calculated absorption spectra of the two mononuclear complexes confirms that Ru can act as a donor to a Co acceptor (Figure S5, Supporting Information). Ru has been well studied76 and is a well-known light-harvesting complex having broad absorption in the visible region, and the Co absorption spectrum confirms that it will not be photoexcited, J


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and 1Co(III) have a combined state energy of 2.10 eV which compares very well with the Ru−Co 3MLCT state, identified as 3 Ru(II) −1Co(III), that has an equivalent 2.11 eV; i.e., the uncoupled pair and weakly bound complex both show the same electronic structure and have very similar energies. These triplet MLCT states are similar to the first excited state observed by time-resolution spectroscopy, and also fall just below the experimental probe energy of 2.35 eV.47 Analogously, an unbound pair of a Ru oxidized doublet and Co reduced doublet and the Ru−Co 3CS state have energies of 0.76 and 0.72 eV, respectively. Finally, the combined energy of the mononuclear 2 Ru(III) and 4Co(II) is 0.20 eV, which compares very well with the Ru−Co 5CS state, having an energy of 0.18 eV and the same charge/spin combinations. It is not surprising that no Ru−Co state corresponding to the 1.08 eV 3[1Ru(II)|3Co(III)] state could be optimized, since there is another lower energy triplet of Ru−Co that was found. Akin to the comparison of unbound [Ru|Co] and weakly bound RuCo, the energies of the different D−A states of the strongly bound RuCo system can also be compared to the uncoupled [Ru|Co] and weakly coupled RuCo systems. However, the RuCo system shows major differences compared to the [Ru|Co] and RuCo systems. In general, the energies of RuCo are lower than the ones of the uncoupled pair (Figure 12 and Table 6), and thus also compared to the RuCo system. In addition, RuCo has a further lower energy state, 3[2Ru(III)4Co(II)], in which a spin flipped 2Ru(III) combines with the lowest energy Co state found for all systems, 4Co(II), as also observed in experiments.46,47 All in all, as discussed in the Results section and supported by the comparative energies of all the D−A states shown in the electronic states Jablonski diagram (Figure 12), the bimetallic RuCo system is a weakly coupled system in which the two metal cores largely behave as two independent sites similar to the unbound [Ru|Co] system. In contrast, the bridge plays a much more significant role in the strongly bound RuCo system, giving typical characteristics of a strongly coupled system. Detailed investigations of energetically accessible manifolds of relaxed excited states in bimetallic systems, as carried out here, supply a basis for more comprehensive understanding of complex excited state decay processes starting from an initially photoinduced excited state that goes beyond a traditional twostate perspective of donor−acceptor systems. Interestingly, this provides considerable opportunities to complement emerging experimental possibilities to visualize structural and electronic dynamics in directly comparable systems.83 In addition, with emerging experimental possibilities to investigate both electronic (spectral) and structural evolution in metallic complexes using time-resolved spectroscopy techniques, identifying the structure, energies, and natures or transient species might soon be possible. However, these experimental methods are expensive and the many overlapping states and time scales make analyzing the results difficult. This is an area that computational studies like this one can suggest types of states and structures that might be present during the excited state evolution. In addition, it is also interesting to consider the influence of the varying degrees of bimetallic coupling in the three systems on the excited state evolution in terms of available excited state evolution, kinetics, and dynamical relaxation. While a full consideration of dynamical processes or advanced coupling calculations between different electronic

Figure 11. Computed spectra of Ru, RuCo, and RuCo with TDDFT PBE0/SDD[Ru,Co]/6-31G[N,H,C]/PCM(MeCN). The complete list of calculated transitions is available in Tables S4, S7, S10, and S13 (Supporting Information) as well as the NTO analysis for the first transitions and those in the MLCT region of significant strength (Tables S6, S9, S12, and S15, Supporting Information). In addition, the first 10 CAM-B3LYP excitations for each complex are in Tables S5, S8, S11, and S14 (Supporting Information).

since its maximal features are far in the UV. The first calculated transition of the mononuclear Ru (underlying lines in Figure S5A, Supporting Information) occurs at 454 nm, in the visible correlating well with the experimental81,82 MLCT peak maximum of 450 nm. The RuCo calculated spectrum has an additional peak at ∼500 nm arising from the lower lying bridge π* and the resulting Ru(d) → bridge(π*) absorption, in exactly the 440−550 nm region where it is observed in experiment.47 Either the Ru MLCT or the excited conjugated bridge can transfer energy to the Co site in a downhill process. On the basis of the energies and electronic characteristics of the relaxed excited charge/spin states, it is clear that the presence of the Co acceptor site in the bimetallic systems opens up new deactivation pathways from the initially excited Ru donor 3MLCT state that involve either electron or energy transfer to the Co site. Table 6 lists the possible excited state combinations for D−A systems introduced in Figure 2, together with the energies of the weakly and strongly coupled pairs. Table 6. Summary of D−A System States for the Unbound [Ru|Co], Weakly Bound [RuCo], and Strongly Bound [RuCo] Systems [Ru|Co]

E (eV)

[RuCo]

E (eV)

[RuCo]

E (eV)

[ Ru(II)|1Co(III)] 1 1 [ Ru(II)*|1Co(III)] 3 3 [ Ru(II)|1Co(II)] 3 1 [ Ru(II)|3Co(III)] 1/3 2 [ Ru(III)|2Co(II)] 3/5 2 [ Ru(III)|4Co(II)]

0 2.73 2.10 1.08 0.76 0.20

GS 1 MLCT 3 MLCT

0 2.74 2.11

GS 1 MLCT 3 MLCT

0 2.41 1.76

3

0.72

3

0.67 0.13 0.33

1 1

CS

CS CS′ 5 CS 3

5

CS

0.18

For all three systems, the overall ground states (with energies set to zero as a reference for all other states) comprise similar singlet Ru and Co sites. Comparison of the [Ru|Co] and Ru− Co relaxed state energies shows a very good match between the uncoupled and weakly coupled systems for each of the corresponding excited states that were found. For the initially formed excited donor state (D*−A), the uncoupled 3Ru(II) K


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Figure 12. Electronic states Jablonski diagram of the various spin state minima found for the unbound [Ru|Co], weakly bound [RuCo], and strongly bound [RuCo] systems.

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states at their crossing points are of interest for applications, these factors clearly fall beyond the scope of the present paper. Focusing on the correlation between structural and electronic relaxation of key charge/spin states that are energetically within reach during photoinduced excited state evolution is a stepping stone toward a more comprehensive understanding of these complex heterobimetallic systems. Some clear qualitative similarities and differences relevant for the excited state evolution between the investigated systems are already emerging at this stage. For all three systems, the calculations confirm that, while initial excitation by visible light, which is a vertical process from the ground state geometry, takes place as an MLCT excitation at the Ru donor site, structural relaxation around the metal cores provides a significant driving force for the formation of low-energy charge separated states involving an oxidized 2Ru(III) and a reduced 4 Co(II) as the final state of the excited state evolution prior to eventual ground state recovery. There is, however, a significant difference for the strongly coupled RuCo system compared to the decoupled [Ru|Co] and weakly coupled RuCo systems in terms of the electronic coupling between the two metal cores being facilitated by strong delocalization of the excited ligand π* onto the bridge already in the initially photoexcited 3MLCT D*−A state. This qualitatively alters the intrinsic nature of the charge separation process from a weak to a strong coupling case. The calculated cross-energies provide some further information about possible intermediate steps in the excited state evolution, although the presented projected potential energy surface picture is of course intrinsically quite limited compared to the full multidimensional excited state evolution. It is, for example, interesting to speculate that the presence of a low-energy quintet state, 5[2Ru(III)4Co(II)]5+, seen at the relaxed 3MLCT geometry in RuCo suggests that there is a competition between activated decay on the triplet surface with direct electron transfer involving an ISC, either resulting in a charge separated state. Such different paths could potentially be discriminated between experimentally, e.g., by investigating differences in the structural relaxations on the PESs of the different electronic states using emerging methods like the current X-ray techniques.

CONCLUSION

Here a thorough computational investigation of relaxed excited states of a set of three heterobimetallic Ru−Co D−A systems characterized by different bridge coupling between the two metal sites has been carried out. Spurred by recent progress in the accurate characterization of excited state relaxation in mononuclear transition metal complexes beyond the Franck− Condon region, this study aims to bring a similar level of improvement in understanding of the structural changes for different charge/spin states encountered during the excited state evolution in realistic heterobimetallic prototype complexes for photoinduced charge separation in molecular solar energy conversion applications. Although these systems have a highly complicated geometric and electronic structure, by correlating the structural changes with the electronic structure of each site and the complex as a whole, this study provides a first step toward understanding the complicated excited state evolution of these systems. By successfully identifying the main reaction coordinates and the related bond displacements with changes in the charge and spin distributions, evidence of charge transfer in both a weakly and strongly coupled system has been observed. By studying two complexes with differing amounts of structural coordination between the two metal sites, the influence of the bridging motif has been examined. The saturated bridge does not interfere significantly with the autonomy of the two metal sites, leading to a chemically bound but electronically only weakly coupled system. In contrast, the conjugated bridge both introduces bridge states that are involved in the initial formation of the excited donor species on the Ru site and increases the coupling between the two metal sites. The approach adopted here, of comparing the relaxations around the two metal cores in the heterobimetallic system with the corresponding mononuclear complexes, has allowed several mixed site excited states to be deconvoluted into their constituent parts. Thus, by utilizing accurate DFT structure optimization methods to investigate several charge/spin states of the three bimetallic systems, a better understanding of the excited state evolution of these systems was achieved. L


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ASSOCIATED CONTENT

S Supporting Information *

Additional computational details including the spin densities and calculated absorption spectra for each of the mononuclear complexes as well as additional excited states explored to aid in identifying the bimetallic excited states. Expanded electronic structure diagrams, the complete list of TD-DFT calculated transitions, and natural orbital analysis (NTOs) of each of the complexes, as well as M06 and CAM-B3LYP energies of optimized minimum geometries and the first 10 TD-DFT transitions with CAM-B3LYP. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Crafoord Foundation, the Swedish Research Council, and the Knut and Alice Wallenberg Foundation. K.K. also acknowledges the support of the Erasmus+ program. We also acknowledge support from the NSC and LUNARC supercomputing facilities.

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REFERENCES

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